1. Field of the Invention
The present invention relates to manufacture of charge detecting devices with a high sensitivity, and more particularly to manufacture of devices such as CCD image sensors which are required to sense micro signal charge, with a high sensitivity.
2. Description of the Prior Art
Conventionally, there have been known various charge detecting devices. The following description will be made, in conjunction with conventional charge detectors employed in solid state image sensors of charge coupled devices (CCDs).
FIG. 5 is a schematic sectional view of a floating surface detector disclosed in "1989's International Conference on Solid State Device and Materials (Extended Abstracts pp. 355 to 358)" by Ohsawa, Toshiba Company, Japan. The floating surface detector comprises a n type substrate 1 and a p.sup.- well layer 2 formed over the surface of substrate 1. Formed on the p.sup.- well layer 2 are a sensing channel 5 which is a high concentration n type region, and a source 20 and a drain 21 both of which are p.sup.+ regions. The floating surface detector also comprises a gate oxide film 4 formed over the entire exposed surface of the resultant structure, a floating gate 12 formed on a portion of gate oxide film 4 disposed over the sensing channel 5, a thickened insulating layer 15 formed over the floating gate 12, and a bias gate 14 formed over the insulating layer 15.
The sensing channel 5 is connected to a CCD which transfers signal charges in a direction normal to the plane of FIG. 5. The signal charges are transferred to a buried channel (not shown). As the transferred signal charges are accumulated in the sensing channel 5, the potential distribution in the sensing channel 5 is varied, so that the potential at the boundary surface between the gate oxide film 4 and the sensing channel 5 and the potential of the floating gate 12 are varied. By these variations, a transistor is activated, which utilizes holes as carriers and the boundary surface between the gate oxide film 4 and the sensing channel 5 as the p type surface channel.
This transistor can operates with a high sensitivity, in that it can have a very reduced sensing capacitance and little parasitic capacity.
The sensing channel also achieves the signal transfer under a completely depleted condition, thereby enabling a complete transfer of signal charges. Accordingly, it is possible to greatly restrain 1/f noise.
Toshiba company reported that the high sensitivity of 76.mu.V/e.sup.- could be compatible with the low noise of no more than 64.mu.V.rms (noise equivalent signal--1.2 electrons rms). However, there is a problem that the source and the drain of the transistor have the same conductivity as the p well, whereas the carriers have a polarity opposite to that of signal charges.
Since the p well is generally grounded, holes present in all regions of the well are diffused into the source or the drain. As a result, a short noise of dark current occurs undesirably due to the diffused holes.
Where an incident light is present, the number of holes generated is the same as the number of electrons. Although an overflow drain is provided for electrons, there is no drain structure which can operate surely for holes.
At the peripheral portion, contacts for the p well are present. However, since the contacts are apart from the pixel portion at a long distance, there is a possibility that a phenomenon of an increase in signal current occurs due to leakage currents of holes which increase temporarily when light is incident.
For solving this problem, a method of using a p.sup.+ type substrate to fix the vertical over flow drain structure for electrons or of keeping the well of charge detector independently from the pixel portion may be assumed. However, it is hardly possible to realize such a method.